The present invention relates to an optical low pass filter (OLPF) provided in an imaging apparatus such as a digital camera.
Recently, digital cameras have been widespread. For digital cameras employing solid-state imaging elements, such as a CCD (Charge Coupled Device), used as an imaging surface, it is important to avoid a moiré effect or an occurrence of false colors.
For this purpose, an optical low pass filter (OLPF) is generally provided between a photographing lens (i.e., an imaging optical system) and the imaging surface so that the high spatial frequency components are removed from an image formed on the imaging surface.
Generally, the imaging element, or the CCD is formed to have a rectangular shape, and a plurality of pixels are arranged at regular interval, in a matrix, along longer and shorter sides of the rectangular shape. In this specification, the term “horizontal direction” refers to a direction corresponding to the longer side of the rectangular imaging element (e.g., CCD), and the “vertical direction” refers to a direction corresponding to the shorter side of the imaging element.
A conventional OLPF is configured to have three cemented birefringence plates (which will be referred to as a three-element OLPF), or two birefringence plates with a predetermined wavelength plate sandwiched therebetween.
A Japanese Patent Provisional Publication No. 2000-56268 (hereafter, referred to as a document 1) discloses a conventional three-element OLPF. The three-element OLPF disclosed in the document 1 has three birefringence plates having separation directions of 0°, +45° and −45°, respectively with respect to the horizontal direction. In general, the separation direction in which a divided ray is directed is represented by an angle with respect to the horizontal direction.
FIG. 9 is an MTF (Modulation Transfer Function) map illustrating an effect of the conventional three-element OLPF. In FIG. 9, a horizontal axis and a vertical axis indicate normalized values of spatial frequencies. Specifically, the horizontal axis in the MTF map of FIG. 9 represents the spatial frequencies in the horizontal direction (X direction), and the vertical axis in the MTF map of FIG. 9 represents the spatial frequency in the vertical direction (Y direction). In FIG. 9, a region A transmits light with a highest transmittance (MTF value: 0.8-1), a region B has a second highest transmittance (MTF value: 0.6-0.8), a region C has a third highest transmittance (MTF value: 0.4-0.6), and a region D has a fourth highest transmittance (MTF value 0.2-0.4). A region E hardly transmits light (MTF value: 0.0-0.2). Note that the definition of the regions A-E applies in all the MTF maps in this specification.
In the conventional OLPF having a characteristic shown in FIG. 9, each of the regions A-D is formed substantially symmetrically both in the vertical direction and horizontal directions. With this configuration, the high spatial frequency components can be eliminated both in the vertical direction and in the horizontal direction in a similar manner. The way the transmitting regions (i.e., the regions A-D) expand will be referred to as a cut-off directionality. The conventional OLPF having the characteristic shown in FIG. 9 has an excellent cut-off directionality such that the characteristic thereof has little direction dependency.
As shown in FIG. 9, the region E in the three-element OLPF disclosed in the document 1 is relatively small. That is, the normalized frequency component of −0.4 or less, or +0.4 or more is not sufficiently suppressed. The function of suppressing/eliminating the high spatial frequency components provided by the OLPF will be referred to as a cut-off function.
As described above, the three-element OLPF disclosed in the document 1 does not have a sufficient cut-off function, although the three-element OLPF has an excellent cut-off directionality. Therefore, the three-element OLPF passes undesired frequency components, which may deteriorate the quality of the image.
A Japanese Patent No. 2840619 (hereafter, referred to as a document 2) discloses a conventional three-element OLPF in which three birefringence plates having separation angles of −45°, 0° and +45°, respectively are provided.
FIG. 10 shows the MTF map of the three-element OLPF disclosed in the document 2. By making a comparison between FIG. 10 and FIG. 9, it is understood that the region E of the OLPF shown in FIG. 10 is relatively large, i.e., the OLPF shown in FIG. 10 has an excellent cut-off function.
However, the regions A-D expand greater in one direction (the direction PL in FIG. 10) than another direction (the direction PS) which is perpendicular to the direction PL. When this OLPF is used, the degree of blur in the PL direction is smaller than the degree of blur in the PS direction. That is, the cut-off directionality of the OLPF in document 2 is inferior to the cut-off directionality of the OLPF in document 1.
In the following description, when the MTF maps are referred to, the direction in which each region (A, B, C and D) expands greatest is indicated as the direction PL, and the direction in which each region expands smallest is indicated as the PS direction.
As described above, the three-element OLPF disclosed in the document 2 does not have a sufficient cut-off directionality, although it has an excellent cut-off function. Therefore, when the three-element OLPF of document 2 is employed, although the excellent cut-off function is expected, due to the lopsided cut-off directionality, the quality of a captured image is lowered since the resolution of the captured image differs depending on the direction.